For yield improvement in veneer peeling by a rotary veneer lathe, it is important that a log should be peeled down to as small a diameter as possible. However, the presence of a pair of spindles engaged with the log at its opposite ends for supporting and driving the log prevents it from being peeled smaller than the diameter of such spindles. To overcome such a problem, a centerless rotary veneer lathe has been proposed in the art which is operable with the spindles retracted away from the log ends from the beginning or in the middle of veneer peeling operation and supports the log by any appropriate means other than the spindles.
One such centerless veneer lathe is disclosed, for example, by U.S. Pat. No. 5,141,038 assigned to the same assignee. This veneer lathe includes a knife carriage carrying a veneer peeling knife and a peripheral drive system for rotating the log from its periphery, and a rotatable backup roll disposed on the opposite side of the log from the knife carriage and contactable with the log peripheral surface during veneer peeling operation for supporting or backing up the log on the above opposite side of the log and also for measuring the current log peripheral speed. The lathe further includes a plurality of backup rolls provided below the log and movable into contact engagement with the log periphery so as to support the log from its bottom.
During initial peeling operation of this centerless veneer lathe, the spindles are engaged with the log at its axial ends and the knife carriage is moved toward the log axial center at a variable feedrate controlled in accordance with the spindle speed so that the knife on its carriage cuts into the rotating log for a predetermined depth for each turn of the log. At any convenient time during the peeling operation, e.g. at a time just after the log has been rounded to become substantially cylindrical, the backup rolls are brought into contact with the log periphery and the spindles are then retracted from the log axial ends. After such contact engagement of the rolls, they are advanced toward the log axial center at the same rate as the feedrate of the knife carriage so that the rolls maintain engagement with the log periphery for measuring the log peripheral speed while backing up the log. After the spindles have thus retracted, the knife carriage feed is effected according to the current log angular speed which can be figured out by a control apparatus from the current log peripheral speed and the current log diameter which can be determined from the current knife carriage position.
This prior centerless veneer lathe is disadvantageous in that it has a plurality of backup rolls all of which need be controlled so as to move synchronously with the knife carriage and, therefore, the lathe becomes complicated in structure and hence costly and also troublesome in the maintenance of the lathe.